Conventionally, devices that estimate a road slope based on the driving force for driving wheels and vehicle acceleration have been proposed as this type of road slope estimating devices (see, e.g., Patent Document 1). Such a device includes a number-of-rotation sensor (rotational speed sensor) that detects the number of rotations (rotational speed) of a turbine shaft as an input shaft of an automatic transmission. The device calculates the driving force for the driving wheels by subtracting gear loss from turbine torque (input torque) and multiplying the resultant difference by a gear ratio, and calculates the vehicle acceleration by dividing a deviation of a turbine rotational speed received this time from a turbine rotational speed received last time by a current gear ratio of the automatic transmission when shifting is not being performed. When the shifting is being performed, or before a predetermined time passes after the shifting, the device maintains the vehicle acceleration that was set last time.